Abstract Type 1 diabetes is one of the most common chronic diseases among children. Managing type 1 diabetes is a complicated, labor intensive effort as one has to monitor blood glucose levels throughout the day, monitor diet, exercise, inject insulin, and adjust insulin based on these activities. The failure to engage in appropriate self- care behavior is problematic as uncontrolled blood sugars can pose both short-term and long-term threats to health. Adolescence is characterized as a developmental period in which youth with type 1 diabetes face difficulties in maintaining good glycemic control, in part due to declines in self-care behavior. Much of the research aimed at this decline has focused on the family, but peer relationships are another important and neglected aspect of youth's social environment. Thus, the present proposal focuses on the role that friends play in the psychological, behavioral, and physical health of adolescents with type 1 diabetes. We have three specific aims: (1) to examine the relations of friend integration and friend conflict to psychological well-being and diabetes outcomes (self-care behavior, glycemic control); (2) to identify potential mechanisms that explain the link of friend integration and friend conflict to these outcomes; (3) to examine the trait of unmitigated communion (i.e., overinvolvement in others to the neglect of the self) as a moderator of the relations of friend integration and friend conflict to outcomes, predicting that links will be stronger for those high in unmitigated communion. We hypothesize that friend conflict will lead to increased psychological distress, distraction from self-care, failure to discuss diabetes, and the perception that friends will respond negatively to the enactment of diabetes self-care. Because the literature on the positive aspects of friendship to diabetes outcomes is mixed, we examine mechanisms for potential positive and negative relations. To the extent that friend integration is beneficial, potential mechanisms include positive affect, disclosure to friends about diabetes, and the perception that friends will respond positively to self-care. To the extent that friend integration is detrimental, potential mechanisms include vulnerability to friend influence and distraction. Because peer relationships occur in the context of family relationships, we also will examine the synergy between the two relationships. We will enroll 160 teens with diabetes in 9th through 11th grades and conduct an ecological momentary assessment (EMA) study over a 4-day period in which interactions with friends and family are assessed as well as previously noted mechanisms. EMA is an innovative method that allows one to tap ongoing experiences as they naturally unfold and to examine within-person variability in links of friend relations to outcomes. The procedure will be repeated 6 months later at yearend. Survey measures of variables will be assessed as a 2nd approach to address the same questions. Regression, multi-level modeling, mediation, moderation, and mediated moderation will be used. The research is highly significant in focusing on friends, using multiple methods, creating an innovative measure of friend knowledge, and employing a strong focus on mechanisms. 1